The present invention relates generally to the packaging of integrated circuits and more particularly to bonding wires to integrated circuit bonding pads.
Semiconductor integrated circuits are currently mass produced for a broad range of purposes. Therefore, continual cost and quality improvements in their manufacturer are valuable and could have broad applicability.
In recent years, multi-chip modules (MCM's) or multi-chip packages (MCP's) containing more than one integrated circuit die have become widely used because complex components can be made by simply connecting multiple dies to each other within a single package. The terms multi-chip module (MCM) and multi-chip package (MCP) will be considered synonymous throughout this application.
To connect dies to each other in MCP's, one must connect the bonding pads of different dies. The ball-stitch technique has been of limited use for bonding wires with both ends connected to integrated circuit bonding pads because capillary tips are typically larger in diameter than the bonding pads. The capillary tip can crack the protective passivation layer(s) surrounding the pads and coating the chip thus permitting corrosion of the integrated circuit underneath. The tip itself can also damage the integrated circuit while cracking the passivation layer(s).
A conventional attempt to use ball-stitch bonding to connect integrated circuit bonding pads to each other is illustrated in FIGS. 1-3. FIG. 1 shows a bonding capillary tip 10 forming a ball bond 15 on a bonding pad 20. The bonding capillary tip 10 has a face 25 with a face surface. The face 25 surrounds an opening of the capillary through which a wire bonding material exudes to form the ball bond 15. Gold is a typical wire bonding material used for making ball bonds.
The bonding pad 20 resides at a surface of an integrated circuit chip 30. Although the bonding pad 20 is shown as having only two layers, it may normally have four or five layers. A layer of the bonding pad 20 may typically be made of an aluminum/silicon compound.
In addition to the bonding pad, the integrated circuit chip 30 has a passivation layer 35 on its exterior. The passivation layer 35 protects the integrated circuit underneath from damage and corrosion. If the passivation layer 35 were to become cracked or damage, the circuit on the integrated circuit chip 30 would be exposed. Additionally, the act of damaging the passivation layer 35 could very well damage the substrate beneath the passivation layer and therefore damage the integrated circuit directly.
After forming the ball or first bond 15 of a bonding wire as shown in FIG. 1, a bonding wire 45 is exuded from the bonding capillary tip 10 as it is positioned in proximity to second bonding pad 40 as shown in FIG. 2. The bonding wire 45 trails out of the opening 50 of the bonding capillary during this positioning step. Generally, in the construction of MCP's, one wants to connect bonding pads of distinct integrated circuit dies. Therefore, the second bonding pad 40 has generally been on a second integrated circuit chip 55 with its own passivation layer 60.
FIG. 3 shows a conventional attempt to make a stitch bond on the second bonding pad 40. Notice that the face 25 of the bonding capillary tip 10 has penetrated the passivation layer 60. Thus, the ultrasonic welding of the bonding wire 45 to form a stitch bond 65 on the second bonding pad 40 will leave a damaged passivation layer 60 with the concomitant problems of an exposed integrated circuit on the chip 55.
A better method for welding bonding wires to bonding pads with both ball and stitch bonds is desirable. Such a method would permit the usage of a single wire bonding machine to create both ball and stitch bonds with the same bonding wire. This transference of a technique previously used primarily to connect integrated circuit chips to lead frames will allow one wire bonding machine both to connect multiple integrated circuit chips to each other and to connect chips to a lead frame. If this new technique were to leave the passivation layers intact, the corrosion and other damage problems caused by current attempts to stitch bond to bonding pads would be obviated.